Effect of Roughness and Contact Coefficients on the Dynamic Behavior of Droplet Impact onto a Solid Surface

In this paper, a lattice Boltzmann model (LBM) is applied to simulate the dynamic behavior of liquid droplets on a solid substrate with variable wettabilities and different micro-topographical patterns. The relationship among fluid/solid interaction parameter, contact angle and surface wettability is obtained by the linear fitting of the pre-simulation results. In the case of 0.314 < |Gt| < 0.563, 0.149 < |Gt| < 0.314 and 0.066 < |Gt| < 0.149, the contact angle will be θ < 90°, 90° < θ < 150° and 150° < θ < 180°, namely the material is the hydrophilic, hydrophobic and superhydrophobic, respectively. Flowing state of droplets on different surfaces are presented. For the hydrophilic rough surface, the droplet fills up the grooves among roughness bumps, while for the hydrophobic one, liquid drop suspends and sits on the tops of the posts with pockets of air beneath it. Most of all, the effects of roughness on droplet behavior are studied and some comprehensive results have been obtained. When the surface is hydrophilic and super hydrophobic, changing the structure of rough units has less impact on the steady-state flow pattern of droplet. And for hydrophobic surface, the effect of surface roughness on the impact characteristics is significant. Only when the structures of rough units meet that the height of the rough unit is greater than the depth the droplets dipping into grooves, droplets can suspend on the top of the rough units with no contact with the bottom wall of the groove.